Why use JMX over simple HTTP servlets?
From my standpoint, JMX is better for 3 reasons:
- It requires less code to enable monitoring points.
- It deals with Java serialized objects end-to-end so there is better data consistency.
- It works with programs that are not servlet based (as you mentioned).
JMX offers a much easier interface to specific data items. You can certainly write the same functionality in a number of servlets, but it is easier for me to expose those using JMX.
For example, if you are using Spring then you can use the org.springframework.jmx.export annotations (@ManagedResource, @ManagedAttribute, etc.) to mark up your classes. I've also published my SimpleJmx framework so you can easily expose attributes and operations with just a couple annotations independent of Spring. For example:
@JmxResource(domainName = "j256", objectName = "lookupCache")
public class LookupCache {
// this can also be done as @JmxAttributeMethod on the getter/setters
@JmxAttributeField(description = "Number of hits in the cache")
private int hitCount;
...
@JmxOperation(description = "Flush the cache")
public void flushCache() {
...
}
}
I have a fully working example program to see how it works. So all you need to do to expose a value or operation is add an annotation to the class and each attribute and/or method. The code to publish it using SimpleJmx looks like the following. Spring is similar albeit with beans:
// create a new server listening on port 8000
JmxServer jmxServer = new JmxServer(8000);
jmxServer.start();
// register our lookupCache object defined above
jmxServer.register(lookupCache);
To get similar functionality in servlets would take a lot more code than just annotations. That said, there may exist frameworks which provide similar functionality in servlet land that I don't know about.
Some more notes:
- There are probably better monitoring tools that grok HTTP/HTML but there are also a ton of distributed JMX monitoring applications. Probably a toss up.
- Being able to programmatically get objects from JMX servers is a plus as opposed to just strings from a servlet page. SimpleJmx also supports a simple JMX client although better ones exist out there.
- Obviously a lot of other worthwhile data is already published by the JVM by default: VM settings, thread details, memory information, etc..
